1. Field of the Invention
The present invention relates to a method of forming a metal film on a resin substrate, to a semiconductor device having a metal film formed on an insulating resin layer and to a wiring substrate. More particularly, the invention relates to a method of forming a metal film which is formed by plating on the surface of the resin substrate while maintaining an increased adhering force, to a semiconductor device and to a wiring board.
2. Description of the Related Art
Conventional methods of forming a metal film have been applied to, for example, build-up substrates, wiring boards inclusive of a semiconductor package using the above substrate and to semiconductor devices called chip-size packages forming a wiring pattern on a semiconductor element via an insulating resin layer.
When, for example, a wiring pattern is formed on a semiconductor device, a wiring layer is laminated between the insulating resin layers. To form the wiring layer interposed between the insulating resin layers, as described above, a substrate having electric insulation such as of polyimide or epoxy resin is used as an underlying member, a coating is applied thereon or an electrically insulating resin film is adhered thereon with pressure to form an insulating resin layer and, then, an electrically conducting layer is formed by plating or the like on the surface of the insulating resin layer. The electrically conducting layer formed on the surface of the insulating resin layer is formed into a predetermined pattern by etching thereby to form a wiring pattern on the surface of the insulating resin layer. Or, the insulating resin layer is masked in a predetermined pattern and, then, an electrically conducting layer is formed thereon by plating to form a wiring pattern.
When an electrically conducting layer is to be formed by plating on the surface of the insulating resin layer, however, it is a practice to effect the plating after the surface of the insulating resin layer is subjected to a roughening treatment (desmear treatment) in order to enhance the adhesion between the electrically conducting layer that is formed and the insulating resin layer as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-57456. The desmear treatment is conducted by etching the surface of the insulating resin layer by using an etching solution such as potassium permanganate or sodium permanganate.
In this case, the conductor is filled in the recessed portions in the surface of the insulating resin layer that is formed rough as the surface of the insulating resin layer is subjected to the desmear treatment by using an etching solution; i.e., the conductor of the wiring pattern is intimately adhered to the insulating resin layer due to the anchoring action. As the surface of the insulating resin layer becomes increasingly rough, however, precision of the pattern is affected by the roughness of the surface at the time of forming the wiring pattern by etching the conducting layer, causing a problem in that it becomes difficult to precisely form very fine wiring patterns.
In forming the wiring pattern by etching the conducting layer depending upon the degree of roughness of the surface of the insulating resin layer, the biting amount increases with an increase in the roughness of the surface of the insulating resin layer, and the biting amount decreases with a decrease in the roughness of the surface. That is, when the surface of the insulating resin layer is very rough, the etching solution easily inters onto the side surfaces of the wiring pattern through the rough portions at the time of forming the wiring pattern by etching the conducting layer, and the side surfaces of the wiring pattern assume a floating state. When the surface is very rough, therefore, it becomes difficult to finely form the wiring pattern.
When the surface of the insulating resin layer becomes increasingly rough, further, there occurs a problem of an increase in the transmission loss of high-frequency signals. The transmission loss decreases with a decrease in the roughness of the surface. As the surface roughness of the insulating resin layer increases, further, the resistance against migration decreases. It is therefore desired that the surface roughness of the insulating resin layer on which the conducting layer is formed is as small as possible. In forming the conducting layer on the insulating resin layer, therefore, it is desired to decrease the surface roughness of the insulating resin layer as much as possible and to improve the adhesion between the insulating resin layer and the conducting layer.
When the conducting layer is to be formed on the insulating resin layer in the semiconductor devices, in general, a metal film is formed, for example, by the electroless copper plating and by the subsequent electrolytic copper plating to thereby form a wiring substrate. The copper plating, however, adheres less to the resin substrate than to the nickel plating. When a metal film that serves as a conducting layer is to be formed by the copper plating, therefore, a further reliable adhesion is required between the metal film and the resin substrate.
In order to improve the adhesion, therefore, there have been proposed a variety of methods of reforming the surface of the resin substrate and, then, effecting the electroless plating on the surface thereof as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-87964. For example, the surface of the resin substrate is irradiated by an ultraviolet light laser, the resin substrate being placed in an atmosphere of an amine compound gas or an amide compound gas, and, then, the electroless plating is effected.
As the pretreatment for effecting the electroless plating onto the resin substrate, there have further been proposed a method of irradiating the surface of the resin substrate with ultraviolet light and, then, effecting the electroless plating on the surface of the resin substrate as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-253869, and a method of improving the adhesion by conducting a step of surface treatment by bringing the surface into contact with an alkali solution containing a non-ionic surfactant having a polyoxyethylene bond as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-88361.
There has further been proposed a method of improving the adhesion of the metal film formed on the resin substrate by electroless plating by reforming the surface of the resin substrate by irradiation with ultraviolet light so as to absorb a silane coupling agent having an amino functional group and by promoting the imparting of a tin-palladium catalyst as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-310873.
In addition to the method of reforming the surface of the resin substrate by irradiation with ultraviolet light, there has further been proposed a method of improving the adhesion by treating the surface of the resin substrate with a plasma and with ultraviolet light in this order, effecting the electroless plating to form a functional group that produces the adhesion to the metal film formed by the electroless copper plating, and decreasing the surface roughness of the resin substrate, as taught in the Japanese Unexamined Patent Publication (Kokai) No. 2002-57456 described above.
When a metal film that serves as a conducting layer is to be formed on a resin substrate which is an insulating resin layer used for a semiconductor device, there have been proposed a variety of methods of improving the adhesion between the resin substrate and the metal film in order to increase the peeling strength that represents the adhesion to a practicable level.
However, the above methods are based on a prerequisite of desmearing the surface of the resin substrate by etching. The etching treatment is generally conducted by immersing the resin substrate in a strongly acidic etching solution such as a mixed solution of chromic acid and sulfuric acid, a mixed solution of dichromic acid and sulfuric acid, chloric acid, or a mixed solution of sulfuric acid and perchloric acid. However, the etching solution is a chemical solution which is dangerous and is highly likely to cause a public hazard, and must be handled and drained with a sufficient degree of attention accounting for a large burden in the operation in the step of plating for forming a metal film.
According to the above methods, further, even though the adhesion can be improved between the resin substrate and the metal film, it is necessary not only to irradiate the ultraviolet light but also to prepare a treating agent for promoting the reformation of the surface of the resin substrate after the irradiation in addition to preparing the treating solutions in the ordinary steps of treatment, causing such problems as an increased number of treatment steps and an increased cost of the treatment.
In improving the surface of the resin substrate, further, when a plasma treatment is to be effected following the irradiation with ultraviolet light, it is necessary to provide a plasma treatment apparatus in addition to the ultraviolet light irradiation apparatus. Provision of the plasma treatment apparatus causes an increase in the cost of facility making it difficult to provide the products inexpensively.